Complex is usable for all cases protect from the wrong polarity DC power supplies. Relay elects towards the voltage power supplies. Needs take into account that contacts of relays must endure current power supplies and also that would be good that have been envisaged for the interruption DC current. The shown connection haves advantage over in respect to the usage diodes:
 there is no the loss of voltages of power supplies
 dissipation is typically 0,5-1W
 protects both ends of power supplies
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